Journal of Lipid Research, Vol 28, 930-940, Copyright © 1987 by Lipid Research, Inc.
Regulation of rat hepatic cholesterol metabolism. Effects of lipoprotein composition on acyl coenzyme A:cholesterol acyltransferase in vivo and in the perfused liver and on hepatic cholesterol secretion
PE Van Zuiden, AD Cooper and SK Erickson
Department of Medicine, Stanford University School of Medicine, CA 94305.
Lipoproteins that are removed from the circulation by the liver can deliver
both cholesterol and triglycerides to the hepatocyte. Relative proportions
of these lipids may vary in lipoproteins and, thus, their uptake may have
differing effects on cholesterol homeostasis. To study this, lipoproteins
containing the same amounts of cholesterol but different amounts of
triglyceride were administered to intact rats or to an isolated perfused
rat liver. The responses of acyl coenzyme A:cholesterol acyltransferase
(ACAT), very low density lipoprotein (VLDL) triglyceride and cholesterol
secretion, and biliary cholesterol content were examined after 2 hr.
Administration of triglyceride-rich chylomicrons (average
triglyceride:cholesterol = 136.5 by mass) in vivo or their remnants
(average triglyceride:cholesterol = 32.7 by mass) to the perfused liver
resulted in an 80% decrease in ACAT activity. In the perfused liver system,
VLDL cholesterol and triglyceride secretion was increased while biliary
cholesterol content decreased. Administration of standard chylomicrons
(average triglyceride:cholesterol = 33.9 by mass) or their remnants
(average triglyceride:cholesterol = 11.4 by mass) lowered ACAT activity by
24% in vivo, but had no significant effect on any of the parameters
measured in the perfused liver system. Administration of cholesterol-rich
VLDL (average triglyceride:cholesterol = 0.47 by mass) in vivo increased
ACAT activity 1.4-fold, but administration of their remnants (average
triglyceride:cholesterol = 0.17 by mass) had little effect on any of the
parameters measured in the perfused liver. Thus, the lipid composition of
lipoproteins removed by the liver elicited acute responses by parameters
important in the maintenance of hepatic cholesterol homeostasis. These
responses reflected the net effects of both the cholesterol and the
triglyceride contents of the particles.